In optical communication systems, a wavelength-division multiplexing (WDM) communication method is employed that transmits optical signals with multiple different wavelengths in one optical fiber. The WDM communication method has increased communication capacity dramatically.
In the WDM communication method, an optical transmitter that allows transmitting of an optical signal with a wide wavelength range is desirable. In order to achieve such an optical transmitter, an optical modulator that operates with high-speed (for example, modulation speed of about 10 Gbps) in a wide wavelength range is desirable.
As a modulator that operates in a wide wavelength range, there is a Mach-Zehnder (MZ) type modulator (Refer to FIG. 1).
In the MZ type modulator, aligning phases between two waveguides that make up an interferometer by manufacturing is difficult due to the influence of manufacturing errors, etc. Thus, some MZ type modulators include not only an electrode for applying a high-frequency signal voltage but also an electrode for adjusting a phase with the length shorter than the electrode for applying a high-frequency signal voltage (Refer to FIG. 2).
In order to adjust chirp, there is a MZ type modulator in which high-frequency electrodes for applying high-frequency signal voltages with different lengths are formed on two waveguides respectively that make up an interferometer (refer to FIG. 3).
The following are examples of related art of the present invention: U.S. Pat. No. 5,991,471 and Japanese Patent Application Publication No. 2007-531022.
As described above, when electrodes for applying high-frequency signal voltages with different lengths are provided, devices with different structures needs to be provided for an devices for zero-chirp operation and for an devices for positive or negative chirp operation. In other words, electrodes with the same length need to be provided as devices for zero-chirp operation on two waveguides that make up an interferometer, and electrodes with the length of one electrode longer than the other electrode need to be provided for positive or negative chirp operation on two waveguides that make up the interferometer.
However, manufacturing of devices takes a few months. Therefore manufacturing of devices with different structures (specification) is not desirable in terms of yield or inventory management.